Heretofore, positive type photosensitive compositions have been widely used in various technical fields as, for example, preparation of semiconductor equipments, printed wiring boards, printing plates and the like. As the photosensitive composition capable of forming a positive image, i.e. positive type photosensitive composition, use has generally been made of such composition as being prepared by adding a quinone diazide compound to an alkali-soluble novolac resin, thereby rendering the composition hardly soluble in a basic aqueous solution to be used as a developer. This system is characterized by making the best use of the nature that the quinone diazide compound used is inherently insoluble in a basic aqueous solution and only soluble in an organic solvent, but when exposed to ultraviolet rays, the quinone diazide group contained is decomposed, thereby forming, passing through a ketene group, a carboxyl group and rendering the compound easily soluble in a basic aqueous solution Examples of such positive type photosensitizers are 1,2-quinone diazide compounds described in patents and other technical publications listed at pages 339 to 357, "Light-sensitive systems", J. Kosar (John Wiley & Sons Inc.). Such positive type photosensitive compositions are, in general, far excellent in resolving power than negative type compositions and this is the main reason why they have been advantageously used as etching-proof materials in the preparation of printed wiring boards and integrated circuit boards. However, in the abovementioned system, there are some problems such that since the novolac resins are prepared by a condensation polymerization process, the products are subject to wide fluctuations in properties and since the softening point is considerably high in despite of a comparatively lower molecular weight of the resin, they are, in general, brittle and also poor in adhesion to a supporting plate when used as a resist film. Therefore, it has long been desired to make up the abovesaid drawbacks. Another proposal has also been made in which a 1,2-quinone diazide compound is admixed with a copolymer of a conjugated diolefinic hydrocarbon, monoolefinically unsaturated compound and an .alpha.,.beta.-ethylenically unsaturated carboxylic acid (Japanese Patent Application Kokai No. 122031/81). In this system, it is said that flexibility and adhesion to a supporting plate is considerably good. However, in order to make this system alkali-soluble by photo-irradiation, it is essential that the content of .alpha.,.beta.-ethylenically unsaturated carboxylic acid in the resin be increased to a considerably higher level, which, in turn, will cause additional problems such that at the developing stage, the unexposed image area is liable to be swelled, thereby lowering the resolving power of the system. It is, therefore, a principal object of the present invention to provide a positive type photosensitive resinous composition which is excellent in flexibility and adhesion, and moreover, capable of resulting a coating whose unexposed area will give the least swelling at the development stage.